Machine vision measuring systems that have more than one camera are used in many applications. For example, wheels of motor vehicles may be aligned on an alignment rack using a computer-aided, three-dimensional (3D) machine vision alignment apparatus and a related alignment method. Examples of methods and apparatus useful in 3D alignment of motor vehicles are described in U.S. Pat. No. 5,724,743, Method and apparatus for determining the alignment of motor vehicle wheels, and U.S. Pat. No. 5,535,522, Method and apparatus for determining the alignment of motor vehicle wheels. The apparatus described in these references is sometimes called a “3D aligner” or “aligner.”
To determine the alignment of the motor vehicle wheels, such 3D aligners use cameras that view targets affixed to the wheels. These aligners generally require a calibration process to be performed after the aligner is initially installed at the work site. In order to accurately determine the position between the wheels on one side of the vehicle and the wheels on the other side of the vehicle, the aligner must know where one camera is positioned with respect to the other camera. According to one calibration method, a large target is positioned in the field of view of the cameras, typically along the centerline of the alignment rack, and away from the cameras. Information obtained from each camera is then used to determine the relative positions and orientations of the cameras. Since each camera indicates where the target is with respect to itself, and since each is viewing the same target, the system can calculate where each camera is located and oriented with respect to the other. This is called a relative camera position (RCP) calibration.
Such calibration allows the results obtained from one side of the vehicle to be compared to the other. Thus, by mounting the two cameras rigidly with respect to each other and then performing an RCP calibration, the system can be used to locate the wheels on one side of the vehicle with respect to the other side of the vehicle from that point on. The RCP transfer function is used to convert one camera's coordinate system into the other camera's coordinate system so that a target viewed by one camera can be directly related to a target viewed by the other camera. One approach for performing an RCP is disclosed in U.S. Pat. No. 5,809,658, entitled “Method and Apparatus for Calibrating Cameras Used in the Alignment of Motor Vehicle Wheels,” issued to Jackson et al. on Sep. 22, 1998.
While RCP calibration is accurate, it requires special fixtures and a trained operator to perform. Thus, there is a need for an easier, simpler calibration process for an aligner.
Further, even after calibration is performed, the aligner may lose calibration over time. The aligner disclosed in the foregoing references has cameras mounted on a boom that is designed to minimize loss of calibration. However, if the cameras are jarred or dismounted, or if the boom itself is bent, the aligner will lose calibration. The aligner cannot detect loss of calibration itself. Loss of calibration normally is not detected unless the technician performs a calibration check or a full calibration. A long time may elapse before the technician realizes that the aligner is out of calibration.
In addition, the boom is large, expensive and presents an obstacle to vehicles entering and leaving the alignment rack. “Drive-through” alignment approaches may be used wherein a vehicle is driven forward into a service facility, aligned, and then driven forward to exit the service facility. This enables other motor vehicles to queue up behind the vehicle being serviced, improving the speed and efficiency of alignment services. In one approach of drive-through alignment that has a rigid boom, it is necessary to raise the camera boom out of the way as each vehicle passes through. This can be time-consuming, costly, and clumsy.
Based on the foregoing, there is a clear need in this field for an apparatus and method that provides for automatic self-calibration of machine vision measuring systems that have more than one camera.
There is also a need for an aligner that may be installed at an alignment service facility without calibration at the installation site, thereby eliminating extra hardware and the need for a trained operator.
There is also a need for an aligner that can automatically re-calibrate itself if its cameras are jarred or dismounted, or if the boom is bent.
There is also a need for an aligner that may be re-calibrated quickly when a technician determines that the aligner was measuring incorrectly, or when a technician suspects that the relative position of cameras of the aligner has changed.
It would also be advantageous to have a 3D aligner that would not require a rigid mounting boom for operation, thereby enabling drive-through alignment without the need to raise the beam and cameras.